The present invention relates to tool arm assemblies of the general type disclosed in U.S. Pat. Nos. 3.190,152; 3,812,747; and 3,232,153, and is particularly concerned with the positioning of a tool, such as a rotary cutting tool, radially of its axis of rotation with a high degree of precision from a remote control station while the tool is being driven in rotation. The present invention provides an apparatus which can accurately adjust the position of such a tool in step-by-step increments of 0.00005 inches or less. The application is a continuation in part of my prior pending applications, Ser. Nos. 135,497 and 193,431 which are incorporated herein for any necessary purpose by reference.